Cryopumps currently available are typically used in equipment for the manufacture of integrated circuits and other electronic components, as well as for the deposition of thin films in a variety of consumer and industrial products. The utility of the cryopumps is to create a contaminant-free vacuum by freezing or adsorbing gases from a work environment.
The design concepts of these cryopumps are similar. The cryopumps comprise a low temperature surface called a primary pumping surface, which operates in the temperature range of 4 to 25 Kelvin (K) and a higher temperature pumping-surface, which operates in the temperature range of typically 70 to 130 K. Typically, a cryogenically cooled radiation shield surrounds the primary pumping surface and provides radiation shielding. Between the chamber to be evacuated and the low temperature primary pumping surfaces is a frontal array at the higher temperature, which serves as a pumping site for the higher boiling point gases and also a radiation shield for the primary pumping surface. The frontal array is typically cooled to 70 to 130 K by thermal coupling it to the radiation shield.
In operation, high boiling point gases, such as water vapor, are condensed on the frontal array. Lower boiling point gases pass through that array and into a volume within the radiation shielding where they condense on the primary pumping surface. Often, an adsorbent, such as activated carbon, is placed on portions of these primary pumping surfaces or other surfaces at temperatures similar to these primary pumping surfaces to adsorb gases which have very low boiling point temperatures and are not condensed on the primary surface. With the gases thus condensed and/or adsorbed onto the pumping surfaces, a vacuum is created.
The refrigerator used for cooling the cryocondensing and adsorbent surfaces may be an open or closed cycle cryogenic refrigerator. The most common refrigerator used is a two-stage cold-finger, closed-cycle refrigerator. Typically, the cold end of the second stage, which is the coldest stage, is connected to the primary pumping and adsorption surfaces. The first stage is connected to the radiation shield which surrounds the primary pumping surface. The frontal radiation shield is cooled by the first stage heat sink through the radiation side shield by means of a thermal path through the complete length of the radiation shield. Typically, the temperature differential across that long thermal path from the frontal array to the first stage heat sink is between 10 and 50 K. Thus, in order to hold the frontal array at a temperature sufficiently low to condense out water vapor, typically less than 130 K, the first stage must operate at between 40 and 100 K.
The heat load which can be accepted by a cryocooler, such as a two-stage refrigerator, is strongly temperature dependent. At high operating temperatures conventional cryocoolers can accept higher heat loads. Thus, a reduction in the temperature differential between the frontal array and the first stage heat sink will allow an increase in the operating temperature of the first stage heat sink. This will allow the cryocooler to accept a higher heat load while maintaining the frontal array at an acceptable operating temperature. To accomplish this reduction in temperature differential, conventional cryopump designs utilize high conductivity materials such as copper in the radiation shields. The gradient can be further reduced by increasing the cross sectional area of the radiation shielding to thus increase the thermal conductance of that shielding. This increased mass of the shielding adds both weight and cost to the product and disadvantageously increases the cool down time and regeneration time of the cryopump.